Choose combustion air method: confined or unconfined space

What is going on

Before anyone sizes combustion air openings, you have to decide whether the furnace sits in a confined or unconfined space. I see this missed on basement and closet installs more than almost anything else, and it is how you end up with spillage, sooting and CO problems. The test is volume against total input of everything burning fuel in that space, not just this furnace. Tight modern houses can behave as confined even when the volume looks big enough.

Likely causes

• Furnace in a closet or small utility room with a water heater or other appliance
• Tightly built house where infiltration will not supply enough air
• Spaces exposed to chemicals, such as pools, laundry or craft rooms, or near chemical storage
• Replacement of an old furnace where the room has since been sealed up

Check in this order

1. Treat the space as confined if it has less than 50 cu ft per 1000 BTU/hr input from all installed equipment
2. For an unconfined space, check whether infiltration is enough. Tight construction may need added air as for a confined space
3. Use outdoor air where chemical exposure is likely, such as commercial buildings, indoor pools, confined laundry or craft rooms, or near chemical storage
4. For a confined space, supply air from inside (two openings to an adjoining room of enough volume)
5. Or supply air from outdoors (two openings, one within 12 in of the top and one within 12 in of the bottom)
6. Check local code for opening sizes, and have a licensed gas fitter confirm the final setup

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)